Process for the doping of silicon



July 21, 1964 E. ENK ETAL 3,141,848

PROCESS FOR THE DOPING OF SILICON Filed June 26, 1961 2 Sheets-Sheet 1 INVENTOR 5101/2/70 awe,

ATTORNEYS July 21, 1964 E. ENK ETAL 3,141,848

PROCESS FOR THE DOPING OF SILICON Filed June 26, 1961 2 Sheets-Sheet 2 \il'gfi l j 15 ATTORNEYS United States Patent 3,141,848 PROCESS FOR THE DQPING 0F SILIQQN Eduard Enk, .Iulius Nickl, and Heinz Silhernagl, Burghausen, Upper Bavaria, Germany, assignors to Wacker- Chemie GmhH, Munich, Germany Filed June 26, 1961, Ser. No. 119,547 Claims priority, application Germany June 24, 1960 1 Claim. (Cl. 252-625) The present invention relates to an improved process for controlled doping of substances.

Considerable difiiculties are still encountered in providing for controlled doping in the production of highly pure substances. For example, great pains must be taken in the production of monocrystalline silicon rods with a reproducible resistance pattern.

It is known, for example, that crystalline substances can be doped during crucibleless zone melting in a protective gas by passing a gas stream carrying doping substances at normal or slightly superatmospheric pressure past the molten zone. In this procedure the gas mixture passes through different temperature zones in which doping substances are already given off. As a result, the concentration of the doping substances along the axis of the rod varies.

According to the invention it was found controlled doping of elements, compounds, alloys and solid solutions can be attained during crucibleless zone melting in gas atmospheres at subatmospheric, atmospheric and superatmospheric pressures with excellent success if rod shaped, liquid or gaseous doping substances, either singly or in admixture, are supplied laterally either continuously or discontinuously to one or more molten zones which move upwardly or downwardly through a vertically arranged body.

The term doping as used herein is not limited to doping as it is known in the semiconductor art but includes the incorporation of substances which are optically, mechanically, magnetically or thermally efiective.

The doping substances which are supplied laterally to the molten zone or zones distribute themselves uniformly in such zone or zones and are built into the solid phase corresponding to the solid/liquid distribution coeflicient and bestow definite properties to the bodies, such as, electrical, magnetic, optical, mechanical or thermal properties, depending upon the type and concentration of the doping substances employed.

In the accompanying drawings:

FIG. 1 diagrammatically illustrates an embodiment of the process according to the invention;

FIG. 2 diagrammatically illustrates a modification of the process according to the invention;

FIG. 3 diagrammatically illustrates another modification of the process according to the invention;

FIG. 4 diagrammatically illustrates a manner in which the doping material in solid rod form can be supplied from a plurality of directions;

FIG. 5 diagrammatically illustrates still another modification of the process according to the invention employing a gaseous doping substance;

FIG. 6 diagrammatically illustrates the manner in which gaseous doping substances can be metered; and

FIGS. 7-12 illustrate various nozzle arrangements for supplying doping gases to the molten zone.

The process according to the invention, for example, can be carried out as illustrated in FIGURE 1 in which a zone 2 of the body to be doped, such as the upright silicon rod 1, is melted and then caused to travel through the length of the rod either upwardly or downwardly. A thin doped silicon rod 5 of a diameter of about 3 mm. is slowly fed horizontally to the molten zone 2 between heating elements 4. As shown in FIGURE 2, the doped "ice rod 5 can also be fed laterally to the molten zone 2 from above. FIGURE 3 diagrammatically illustrates an arrangement for feeding the doping rod 5. In such arrangement the doping rod 5 which is fed from the left is held by clamp 6 which is fastened to a shaftor rod '7 which is driven by rack and pinion 8 and 8'.

The advantage of the process resides. in that the con trolled predetermined doping, such as effecting a definite specific electric resistance in the body to be doped, is achieved by the linear movement of the doping rod with the aid of the feeding arrangement. Linear movements today are a solved problem and can be carried out with the greatest exactitude. It is possible with the process according to the invention to use a doped rod of uniform or non-uniform cross-section as the source of the doping substance as it is easy to ascertain the measurements of the rod and to program the advance of the doping rod to correspond to its doping substance content. The doping rod also can be uniformly or non-uniformly doped.

For example, if the content of a specific doping sub stance in the doping rod decreases from the end held in clamp 6 towards the end supplied to the molten zone, it is simple to adjust the drive so that the rate of feed decreases linearly corresponding to the doping substance content of rod 5. It is therefore possible to provide for a uniform resistance along the axis of rod 1 (FIG. 3). In an analogous manner it is possible to compensate for geometric non-uniformities in the rods, such as variations in diameter, with mechanical measures only. For example, if the doping rod is conical and uniformly doped its feed is so regulated that the quantity of doping substance supplied per unit of time remains constant. This is of special significance when a plurality, for example, several thousand rods with exactly the same properties, such as, for example, specific resistance, are to be proed.

When the molten zone travels relatively quickly through the rod to be doped, it is of advantage to feed two or more doping rods to the molten zone. Such an arrangement is shown in FIGURE 4 in which three doping rods 5 are simultaneously uniformly fed to the molten zone 2.

The process according to the invention furthermore renders it possible by altering the rate of feed of the doping rod or rods to the molten zone to provide for a certain variation of the specific resistance of the body to be doped, for example, its resistance may be caused to decrease from one end to the other.

In addition, zones with difierent types of conductivity can be produced in one and the same body by using doping bodies containing oppositely acting doping substances. For example, if a silicon rod or tube is first doped with a boron containing doping rod, p-conductive silicon is produced. If the boron containing doping rod is then replaced by a phosphorus containing doping rod a zone with n-conductivity will then be produced in the silicon rod or tube. As a consequence, bodies with many zones of opposite conductivity types. can be produced.

By simultaneous supply of oppositely acting doping substances it is possible to obtain a complete or partial compensation of, for example, the type of conductivity. In this way intentionally compensated material can be produced. a

The rods used to supply the doping substances can be of round or angular cross-section and preferably are based upon the same material as that of the bodies to be doped. However, other substances which vaporize or melt and do not disturb the dominating properties of the body to be produced can also be used to supply the doping substances. Silicon rods can, for example, be doped with the aid of silicon rods about 3 mm. in diameter containing doping substances, such as, boron, aluminum, indium, gallium or phosphorus, arsenic and antimony. Analogously, in

doping germanium or boron bodies it is preferable to use germanium or boron rods to supply the doping substances.

The doping bodies can also be produced from a material which takes part in the make-up of the body to be produced and remains therein. For example, it may be an alloying component or an element which forms a compound with the substance of the rod to be doped. For instance, a boron rod containing phosphorus can be employed to dope a silicon rod whereby phosphorus containing borosilicides are produced. Analogously, the process is also suited for the production of intermetallic compounds, such as the III/V-group element, II/VI- group element and V/IV-group element compounds.

The doping bodies can contain the active doping substances in homogeneously or heterogeneously distributed form, for example, they can be in the form of occlusions in solid bodies, or in the form of solid solutions or as dissolved elements or compounds.

A further advantage of the process according to the invention is that it is not bound to any particular type of heating employed to produce the travelling molten zone. It is suited equally well when electric high frequency heating, electron or ion bombardment, a plasma burner, an electron torch or radiated heat is employed for such heating.

The process furthermore is not limited to the use of any particular range of pressures at which the zone melting is effected as it can be carried out at subatmospheric, atmospheric or superatmospheric pressures.

When a gaseous doping substance is employed under vacuum, it is supplied directly to and only to the molten zone with the aid of an appropriate nozzle This is achieved in that the spacing between the nozzle end and the surface of the molten zone is not greater than the free path of the gas particles at the operating pressure. In this way, practically every gas particle reaches the molten zone and the adjacent solid portions remain free of uncontrollable additional doping substances. It is possible to operate without any or with only a relatively small quantity of protective gas.

FIGURE 5 diagrammatically shows an apparatus suitable for carrying out the process according to the invention with gaseous doping substances. In such figure, 9 represents a recipient such as commonly used for zone refining. Rod 1 is first zone refined therein about 5 times either upwardly or downwardly using a high frequency coil 4 as source of heat without addition of the doping gas. Then in the sixth zone refining the doping gas is supplied to the molten zone through conduit 10 during the entire travel of the molten zone.

FIGURE 6 diagrammatically illustrates the manner in which the doping gas is metered. The gas flows from pressure bottle 11 over a pressure reducer 1.2 of known construction and valve 13 into measuring chamber 14 provided with manometer 15. Valves 16 and 17 remain closed and fine regulating valve 1% is opened.

FIGURES 7-12 illustrate various arrangements for nozzles for supplying the doping gas to the molten zone.

In FIGURE 7, nozzle 19 is arranged perpendicularly to the axis of rod 1 and nozzle 20 is arranged at a downwardly directed angle. In FIGURE 8, nozzle 19 is arranged between two heating coils 4 and inFIGURE 9 the double outlet nozzle 23 is arranged to supply the doping gas to the molten zone symmetrically above or below the heating coil.

In prerefining of materials tending to vaporize strongly it is possible that the nozzle serving laterfor supply of the doping gas might become clogged. In such case nozzle 21 is placed in location a (FIGURE 10) during the prerefining and in location b during-the doping. FIG- URE 11 shows an arrangement in which three nozzles 22 are arranged symmetrically around the rod. In FIGURE 12 the nozzle 24 is projected through the high frequency heating coil 41 to provide a completely symmetrical incidence of the gas stream on the molten zone.

It is essential for the process according to the invention that the distance of the nozzle outlet from the surface is adapted to pressures which are operated at. The lower the pressure the greater the spacing may be. However, it should not be greater than the length of the mean free path of the gas particles at the operating pressure. With such mode of operation the doping substances only impinge upon the molten zone.

Volatile or vaporized organic or inorganic compounds, such as, for example, alkyl compounds, aryl compounds, hydrides, halides, oxides, sulfides, selenides and elements either singly or in a mixture are suited as doping substances. They can be supplied to the molten zone either in undiluted form or with the aid of an inert propellant gas. For example, silicon can be doped using boron hydrides, boron halides, phosphorus vapor, phosphine or phosphorus halides as the doping substances.

The molten zones during the process according to the invention may be heated by high frequency electron heating, electron bombardment, plasma burners, radiated heat or focused radiation using stationary or moving heating elements. In the case of plasma burners the doping gas can be supplied directly to the fuel gas.

By regulating the rate of supply of the doping substances it is possible to provide a definite pattern of properties to the doped body, such as, for example, a specific resistance which decreases from one end to the other end of the rod or tube.

By switching over to oppositely active doping substances, zones with opposed properties can be produced in one and the same body. For example, germanium rods can be alternately doped with boron hydride and phosphine so that zones with opposite types of conductivity are obtained.

The simultaneous supply of oppositely acting doping substances renders it possible to provide zones in which the doping substances are partially or completely compensated.

The doping gas or rod need not necessarily be directed against a molten zone which encompasses the entire crosssection of the rod. For example, it is advantageous to supply the doping gas or rod to a molten zone which does not encompass the entire cross-section of the rod and precedes a molten Zone encompassing the entire cross-section of the rod as it travels along the rod. A very good distribution of the doping substances over the entire cross-section of the rod is achieved in this way.

The process according to the invention renders it possible to provide for doping over a wide range of resistances in nand pconductive material, for example, in the case of silicon of about l0 -10 ohm cm. and carrier lives of 10 to several microseconds.

" The process according to the invention can also be applied to zone melting in crucibles.

Example 1 A polycrystalline silicon rod (diam. 15.0-17.3 mm., length 300 mm.) vertically arranged was zone floated in vacuum from 10- to l() mm. Hg. The molten zone was caused to travel 8 times upwardly through the rod with a velocity of 2 mm./min. After the 8th zone passage the rod had a final length of 250 mm. and a diameter of 16.0 mm. This rod was doped in the 9th zone passage by supplying a thin silicon rod (diam. 2010.05 mm, length 350 mm.) to the molten zone. The doping rod had a boron-content corresponding to a resistivity of 7.8 ohm cm. The rate of feed and the velocity of the molten zone in the thick rod was 2 mm./ min.

The wanted resistivity of the doped rod should be 500 ohm cm. It was empirically known that the thick rod had an electrical resistivity of about 8000 ohm cm. after the 8th zone passage. The achieved resistivity over alength of 250 mm. was 470112 ohm cm..

, So it is also possible to dope other elements with doping material in form of rods.

5 Example 2 An aluminum rod (diam. 5 mm., length 200 mm.) was vertically arranged zone refined. The velocity of the molten zone was 1.5 mm./min. To this molten zone an antimony rod (2 mm. diam.) was supplied continuously. The rate of supplying was so that the melt had the composition of AlSb. After that 2 further zone passages were carried out in order to make the material monocrystalline.

We claim:

A process for the controlled doping of silicon during zone melting thereof which comprises causing at least 6 one molten zone to travel axially through an elongated vertically arranged body of silicon and supplying a silicon rod, which contains the doping material laterally to said travelling molten zone at a predetermined controlled 5 rate.

References Cited in the file of this patent UNITED STATES PATENTS 10 2,739,088 Pfann Mar. 20, 1956 2,964,396 Rummel et a] Dec. 13, 1960 2,970,111 Hoffman et a1. Ian. 31, 1961 

